Hiruzen Sarutobi regrets many things.
He regrets his first kill; the eyes of a young Kumo ninja still haunt his dreams. He has forgotten all those after but the young, terrified face of the brown-haired boy from the mountains is forever imprinted in his mind. He can still feel the way the kunai slid across his neck so easily - too easily - like cutting into water. The sounds of the boy choking on his own blood reverberates through his mind to this day. Hiruzen was eight years old when his soul was damned.
He also regrets his acceptance of the position of Hokage. He had been young and foolishly ambitious when the grim face of Tobirama Senju, his sensei and idol, turned to him and asked that Hiruzen succeed him. At the time he felt a conflict of pride and desolation, watching the man who had placed his faith in him sacrifice himself so that he might live. Now, there was no such conflicting feelings - he only felt permanent, unending anxiety and sadness. Those who wore the red-brimmed hat were cursed to either misery or death. Hiruzen wished he received the second.
He regrets being born into the Sarutobi clan. A strong, and proud family, Sarutobi's survived on loyalty and love to one another. He offered neither to them, the position of Hokage stealing all the love in his heart and stripping him bare. His Biwako died as a result of his meddling, his children hated him as a result of his absence and his grandchild, sweet, sweet Konohamaru, lived without a family at all as a result of his actions. Hiruzen hated what the Hokage hat had done to his family but he hated himself even more for it.
He, and his many friends and teammates, greatly regret the faith he had placed in his prodigious students. As they embraced his lessons and soared up the ranks Hiruzen had felt pride. His young children, the cute Tsunade, the cheeky Jiriaya and the innocent Orochimaru, were becoming their own people and he liked what he saw. Ultimately however, like everyone close to him, Hiruzen still let them down. The incredible and talented Orochimaru whom he loved like a son, grew alienated and bitter; consumed by his own genius. The unstoppable and beautiful Tsunade, Hashirama-sensei's final gift to him, loved too much and too hard until finally her heart gave out. The powerful and charismatic Jiraiya, the one he least appreciated and yet the one that made him most proud, couldn't even look him in the eye - his loyalty and passion corrupted by Hiruzen's dark heart. He wished that he had never walked into that classroom half a century ago.
He, and all of Konoha, regrets his retirement. After putting in decades of work for the village Hiruzen had foolishly thought he'd earned a break. As a result of his selfishness the world lost a hero, a genius, someone who could of fixed the sickness that captured the minds of shinobi - someone who could of made a difference. The immeasurable and incomparable Yondaime threw his life away for a duty that should not of even been his. Another life and another family ruined because of Hiruzen. Even now, years later, he sometimes prays and begs the Shinigami to take him instead - as if the gods cared for the troubles of the mortal world.
He regrets his decisions in the aftermath of the Yondaime's death. Hiruzen placed faith in the fickle minds of humanity - in a village he had lived his entire life. He didn't know what was worse, the monstrous mistreatment of his successor's legacy or the monstrous ignorance of himself to believe it would be any different. After all, Konoha was full of people cut from the same cloth as himself. And he dreaded the idea of even one other person existing with the same darkness in their soul as his own.
He, and Minato and Kushina surely, regret abandoning that very same child, leaving him to wallow in the despair and hate of the common man. Whether his ANBU punished those who spoke the words he'd forbidden or whether the terrifying Ibiki tortured those who even dared to lay a hand on the small, so so small, body of the blond-haired Namikaze child it didn't matter - because he was always too late. He watched as the child's smile grew dimmer and dimmer until it struggled to even survive on the chubby, childish face. He watched as the boy became more miserable, more independent and more angry. Hiruzen watched as Naruto - the boy who was surely destined for greatness - lost his soul to the beast that was Konoha.
He, imagining the cursings and disgust of his successor, regretted staying uninvolved in that child's life, in watching him shrivel away into nothing. He watched silently from the sidelines for seven years as the boy was tortured by his own existence and only did the bare minimum. He watched as the child grew restless and aggressive, avoiding anyone and everyone. He did not watch when the boy had decided one day that enough was enough and, when a stupid, idiotic, ignorant man hit him fought back, stabbing a kunai through his skull and running out of the gates of the village into the wilderness.
More than anything though, Hiruzen knew he would regret what he was about to do.
He stared at the two ANBU holding the struggling dirty and ragged blonde-haired seven year old Naruto Uzumaki. He stared at the talented fuinjutsu user beside the boy who was studying the seal on the boy's stomach. He stared at the pure loathing in the beautiful blue eyes of the child - for a moment wondering if he was looking into a mirror. Flanked by his advisors on either side, the ambitious Danzo listening in keenly, Sarutobi Hiruzen spoke the words that he knew doomed him to the very depths of hell.
"Remove the demon," His voice was cold, "Danzo will find you a suitable replacement host." The boy continued to struggled, ignorant that the man he had once called his grandfather had just sentenced him to death. Hiruzen turned to look at Danzo and saw a wicked, evil smile stinging across his face.
Yes, Hiruzen regretted many things but more than anything...
Hiruzen Sarutobi regretted losing his humanity.